The present invention relates to a twin track machine and to a method for manufacturing composite filters attachable to cigarettes, cigars and the like.
Reference is made explicitly to cigarettes in the following specification, albeit with no limitation in scope implied.
Conventionally, the harmful effects of inhaling cigarette smoke are reduced by tipping cigarettes with composite filters, that is to say with filters obtainable by pairing together two or more filter plugs made of material having different filtration properties, which are joined permanently together by being enveloped in a paper plugwrap.
The plugs of different type, or at all events with different filtration properties, are fed by respective trains of rollers to a common take-up and ordering conveyor, furnished with peripheral flutes.
Thus, each of the flutes in question accommodates a single composite filter element consisting in a group of two or more filter plugs having different properties, axially aligned and placed in end-to-end contact.
In twin track machines, to which the present invention relates, these composite filter elements are transferred to a garniture tongue on which two composite filter rods are fashioned.
More exactly, the two successions of filter elements are caused to advance along a direction parallel to their longitudinal axes, and enveloped in respective strips of paper material (plugwrap) to form continuous rods.
At the outfeed end of the garniture tongue, the continuous rods are divided up simultaneously by a single cutter device into a plurality of discrete filters.
To keep the two successions of filters aligned transversely one with another at the moment of the cut, conventional machines comprise means by which to correct the alignment.
In other words, a detection unit is placed in close proximity to the cutter device, such as will compare the positions of the corresponding plugs making up each group in the two rods, both in relation one to another and in relation to the cutter device.
In particular, should the corresponding plugs in the two rods be found to have drifted out of position one relative to another, the correction means aforementioned will be activated.
In one prior art method, the correction is made by adjusting the feed rate of at least one of the strips of paper plugwrap material, and consequently modifying the speed at which one of the two rods is assembled.
In other words, the correction means are designed to accelerate or decelerate the feed rate of the strip of paper material in such a way that the alignment error revealed by the detection unit is reduced to zero.
However, in the event that the misalignment is caused by a cumulative error, then the misalignment will be incremental. In this situation, the magnitude of the correction, which is proportional to the degree of misalignment, will be incremental likewise. Given that in conventional machines, the error is corrected by operating directly on the plugwrap material, the magnitude of the correction can become so great that there is considerable risk of the plugwrap material being broken.
In other words, if the misalignment is generated by a cumulative systematic error, the conventional machines in question are unable to reduce the misalignment to zero. In practice, the corrective action can be applied only to the point of inducing a break in the strip of plugwrap material.
Accordingly, the object of the invention is to overcome the drawbacks described above by providing a twin track machine and a method for manufacturing composite filters attachable to cigarettes, cigars and the like, designed to ensure that systematic and cumulative errors affecting the manufacture of such filters can be successfully corrected.